This invention is directed to jet aircraft and more particularly to controlling the augmenting lift used by upper surface blowing aircraft during low speed operation, such as during take-off and landing.
Various types of short take-off and landing (STOL) and vertical take-off and landing (VTOL) aircraft have been proposed and are in use. In general, these aircraft include some form of means for augmenting or entirely replacing the aerodynamic lift of the wings during low speed, particularly take-off and landing, operation.
One type of aircraft that includes a method of and an apparatus for augmenting the aerodynamic lift created by its wings during low speed operation is an upper surface blowing aircraft. An upper surface blowing aircraft utililzes jet engines mounted so that the jet exhaust occurs above the wings, rather than below the wings, as in conventional aircraft. The upper surface exhaust, during low speed operation, is "turned" downwardly over extended flaps located at the rear of the wing surface. The "turning" of the exhaust occurs without external mechanical means, in accordance with the well known Coanda effect. The downwardly directed exhaust provides the desired lift augmentation during low speed operation, particularly during take-off and landing.
One of the problems with upper surface blowing aircraft relates to the cross-sectional configuration of the jet exhaust. A standard, relatively thick, jet exhaust will not follow the curve created by the extended flap. That is, for the Coanda effect to take place, the negative pressure on the side of the exhaust adjacent to the wing must be adequate to overcome the tendency of the exhaust to follow a straight line. As the thickness of the exhaust diminishes, the amount of negative pressure necessary to "turn" the exhaust also diminishes, assuming that the radius of curvature over which the exhaust is to turn remains the same (i.e., the amount of negative pressure is also inversely related to the radius of curvature). In other words, if the exhaust, vertically, is relatively thick, it will separate from the wing and flap and now flow downwardly as desired. On the other hand, a jet engine exhaust is required to provide satisfactory performance under both low speed (usually take-off and landing) and high speed (cruise) conditions.
Previously, conventional jet aircraft (under the wing exhaust) have been able to obtain acceptable performance under both conditions with fixed exhaust nozzle geometry. However, such is not the case with upper surface blowing aircraft that use a Coanda flap system (a flap system that utilizes a Coanda effect) to deflect the jet engine exhaust downwardly to obtain a powered lift that augments (STOL) or replaces (VTOL) conventional aerodynamic lift. More specifically, as explained above, effective Coanda flow turning can only occur if the height (thickness) of the nozzle flow is limited to a certain percentage of the radius of curvature of the flap. If this limit is not met, the negative pressure naturally occurring on the wing upper surface side of the exhaust will be inadequate to turn the exhaust over the flap. Since flap size limits generally restrict the radius of flap curvature, they consequently limit the maximum exhaust flow thickness which can be turned.
One possible solution of the foregoing problem is to direct the exhaust flow downwardly, at high angles relative to a horizontal plane, by deflectors or nozzle inclination. However, this solution is unsatisfactory because the effective area of the nozzle is low at take-off and high at cruise which can result in engine nozzle mismatch and/or poor performance under high speed (cruise) conditions. Moreover, the nozzle area increase at cruise cuts the boost compressor stall margin. If the nozzle is sized for cruise to overcome this disadvantage, at take-off, an under area nozzle exhaust condition exists which can decrease the fan stall margin to below a minimum acceptable level and/or reduce take-off thrust proportionally to the nozzle area decrease. An alternative solution is to decrease exhaust thickness by shaping the nozzle so that the exhaust flow is spread over a large area of the wing. A very wide fixed nozzle can be utilized to obtain the desired spreading, and resultant thinning, of the jet exhaust. However, such a solution also results in cruise disadvantages. For example, it has been found that a wide nozzle, suitable for low speed performance, has an effective nozzle velocity coefficient which is less than desired during cruise. In addition, a wide nozzle or high aspect ratio nozzle has high cruise drag and high weight. These disadvantages result in increased cruise fuel consumption, and an equivalent loss in range.
Therefore, it is an object of this invention to provide a method of and an apparatus for controlling jet exhaust flow attachment to the wing and flap surfaces of an upper surface blowing type aircraft.
It is a further object of this invention to provide a method of and an apparatus for promoting flow attachment to the wing and flap surfaces of upper surface blowing type aircraft only during low speed flight and not during high speed flight.
It is also an object of this invention to provide an apparatus for controlling the divergence of the jet engine exhaust of an upper surface blowing type aircraft so as to promote Coanda effect flow attachment to the associated wing and flap of the aircraft.
It is a still further object of this invention to provide a new and improved engine, wing and flap combination suitable for use in an aircraft.
It is yet another object of this invention to provide an upper surface blowing engine, wing and flap combination suitable for use in an aircraft.